perf(browser): improve the parallelisation logic

[sheremet-va]

Description

Just experimenting with different methods (this doesn't work yet). Ideally, tinypool should expose a way to do that @AriPerkkio

Related #6834
Related #7616

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[AriPerkkio]

The support from Tinypool's side is quite complex as Piscina's codebase is not flexible. There's some work done on tinylibs/tinypool#98 but I'm not sure when I'm ready to continue that 😱

[sheremet-va]

Follow-up: #7665