Review most used code in terms of performance

[jodator]

📝 Provide a description of the improvement

Doing the MergeCells command PR review @oleq found out that undoing merge of large selection is pretty slow.

It looks like some significant time is spent in code that is not suspicious at a first glance. Some examples are (from initial report):

1. Element's is() comparison is doing string.replace() for model: prefix checking. Adding many checks up this is significant time.
2. Position's constructor is doing Array.concat() that has some performance issues. Changing it to spread operator (as a wild guess - it hasn't that performance boost in synthetic tests) also cut some time.

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I see a potential quick win (spending additional 1-2 hour) in reviewing some repeatable code and checking if it has some potential fixes (ie those that will not change logic or API).

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[jodator]

Repeatable result:

1. Testing done on "Performance: pasting data" manual test using "full websites" setting.
2. Test: invoke "full website" -> click to focus -> CTRL-Z (undo).

Result: 466ms to 87.4ms for the line of concatenating parent path (same time reduction in undo execution).

after:

[jodator]

And another gain by using for loops instead of spread operator: from 56.6ms self time to 3.5ms self time (I had to extract those to methods to compare this part of code).

after:

Side note: because of the time fluctuation I'd only consider an order of magnitude change (as above) to be significant enough.

[Reinmar]

Waiting for the PRs... 🛎

[jodator]

Another finding - the is() method is painfully slow because:

1. It does string operations 
2. Calls super...

Below is a result RootElement.is() times:

Before:

After (removing string concatenation and do not call super.is():

[jodator]

I've double checked the latter finding and this is bonkers:

Current code:

After changes only in RootElment:

The changes:

is( type, name ) {
	if ( !name ) {
		return type == 'rootElement' || type == 'element' || type == this.name || type == 'node';
	} else {
		return ( type == 'rootElement' && name == this.name ) || ( type === 'element' && name == this.name );
	}
}

[jodator]

I've inlined most of the is() checks and I've removed that mode: and view: checks. Gain 10s in the above scenario.

Before (undo callback took 25.1s ):

After (undo callback took 14.89s):