Delay in `runtests-browser`

As of #20763, it's taking a lot longer to bring up runtests-browser. I've tested this in two parallel checkouts, one in #23875 and one in the commit prior to #20763 (24e58c3).
First I ran gulp runtests-browser in both to ensure everything was built. Then running gulp runtests-browser --test findAllRefs in both:

• Update harness to use single robust virtual file system for tests. #20763: It takes about 30 seconds after the tab opens until I'm able to open the debugger.
• Prior commit: it takes only second until I can open the debugger.

There is also a delay when refreshing the window -- so If I missed something and have to restart, I have to wait 30 seconds to begin debugging again.

[mhegazy]

I am seeing that too locally after @rbuckton’s vfs change

[rbuckton]

I can take a look first thing tomorrow. I think it has something to do with caching.

I had exception breakpoints turned on and it also looks like we throw a lot of exceptions in vfs.ts on startup -- not sure if that's the cause though.

[sheetalkamat]

I have had trouble with lot of exceptions being thrown. Everytime i make some change, i need to go ahead and disable the breakpoints at those exception sites.