diff --git a/pycbc/events/coinc.py b/pycbc/events/coinc.py
index f0d98323f91..7b8ee9840d2 100644
--- a/pycbc/events/coinc.py
+++ b/pycbc/events/coinc.py
@@ -35,6 +35,14 @@
 from .eventmgr_cython import timecoincidence_findidxlen
 from .eventmgr_cython import timecluster_cython
 
+# Mapping used in background_bin_from_string to select approximant for
+# duration function, if duration-based binning is used.
+_APPROXIMANT_DURATION_MAP = {
+    'SEOBNRv2duration': 'SEOBNRv2',
+    'SEOBNRv4duration': 'SEOBNRv4',
+    'SEOBNRv5duration': 'SEOBNRv5_ROM'
+}
+
 
 def background_bin_from_string(background_bins, data):
     """ Return template ids for each bin as defined by the format string
@@ -55,6 +63,9 @@ def background_bin_from_string(background_bins, data):
     """
     used = numpy.array([], dtype=numpy.uint32)
     bins = {}
+    # Some duration/peak frequency functions are expensive.
+    # Do not want to recompute many times, if using lots of bins.
+    cached_values = {}
     for mbin in background_bins:
         locs = None
         name, bin_type_list, boundary_list = tuple(mbin.split(':'))
@@ -71,7 +82,9 @@ def background_bin_from_string(background_bins, data):
                 raise RuntimeError("Can't parse boundary condition! Must begin "
                                    "with 'lt' or 'gt'")
 
-            if bin_type == 'component' and boundary[0:2] == 'lt':
+            if bin_type in cached_values:
+                vals = cached_values[bin_type]
+            elif bin_type == 'component' and boundary[0:2] == 'lt':
                 # maximum component mass is less than boundary value
                 vals = numpy.maximum(data['mass1'], data['mass2'])
             elif bin_type == 'component' and boundary[0:2] == 'gt':
@@ -91,34 +104,29 @@ def background_bin_from_string(background_bins, data):
             elif bin_type == 'chi_eff':
                 vals = pycbc.conversions.chi_eff(data['mass1'], data['mass2'],
                                                  data['spin1z'], data['spin2z'])
-            elif bin_type == 'SEOBNRv2Peak':
-                vals = pycbc.pnutils.get_freq('fSEOBNRv2Peak',
-                                              data['mass1'], data['mass2'],
-                                              data['spin1z'], data['spin2z'])
-            elif bin_type == 'SEOBNRv4Peak':
-                vals = pycbc.pnutils.get_freq('fSEOBNRv4Peak', data['mass1'],
-                                              data['mass2'], data['spin1z'],
-                                              data['spin2z'])
-            elif bin_type == 'SEOBNRv2duration':
-                vals = pycbc.pnutils.get_imr_duration(
-                                   data['mass1'], data['mass2'],
-                                   data['spin1z'], data['spin2z'],
-                                   data['f_lower'], approximant='SEOBNRv2')
-            elif bin_type == 'SEOBNRv4duration':
-                vals = pycbc.pnutils.get_imr_duration(
-                                   data['mass1'][:], data['mass2'][:],
-                                   data['spin1z'][:], data['spin2z'][:],
-                                   data['f_lower'][:], approximant='SEOBNRv4')
-            elif bin_type == 'SEOBNRv5duration':
+            elif bin_type in ['SEOBNRv2Peak', 'SEOBNRv4Peak']:
+                vals = pycbc.pnutils.get_freq(
+                    'f' + bin_type,
+                    data['mass1'],
+                    data['mass2'],
+                    data['spin1z'],
+                    data['spin2z']
+                )
+                cached_values[bin_type] = vals
+            elif bin_type in _APPROXIMANT_DURATION_MAP:
                 vals = pycbc.pnutils.get_imr_duration(
-                                   data['mass1'][:], data['mass2'][:],
-                                   data['spin1z'][:], data['spin2z'][:],
-                                   data['f_lower'][:], approximant='SEOBNRv5_ROM')
+                    data['mass1'],
+                    data['mass2'],
+                    data['spin1z'],
+                    data['spin2z'],
+                    data['f_lower'],
+                    approximant=_APPROXIMANT_DURATION_MAP[bin_type]
+                )
+                cached_values[bin_type] = vals
             else:
                 raise ValueError('Invalid bin type %s' % bin_type)
 
             sub_locs = member_func(vals)
-            del vals
             sub_locs = numpy.where(sub_locs)[0]
             if locs is not None:
                 # find intersection of boundary conditions