Startup time is too long

[nortakales]

It was mentioned in #32 that startup time for the extension can be upwards of 20 seconds. I'm personally seeing around 15 seconds. This is too long and should be investigated.

[tylerlaprade]

This is by far my slowest extension, but it's so useful that I don't want to disable it. Based on the linked issue above, if this is indeed due to the currency exchange rate API, I'm happy to just disable that if possible since I tend not to do anything with currencies.

[nortakales]

At some point I had added some instrumentation to see where the time was being spent during startup, but I don't remember finding anything conclusive. That was quite a long time ago at this point though, so I definitely need to tackle that again. It can't be from fetching the exchange rates because the vast majority of the time it will grab them from a local cache. Especially if you just reload VSCode, it will definitely have cached values (but still takes a while for the extension to start).