Learn Finnish: Tämä kallis tuote toimii hyvin hitaasti. - This expensive product works very slowly.

To make an adverb, you attach the -sti suffix to the word's vowel stem (the same base you use before adding most endings). For words ending in -as like hidas, the stem changes to -aa- (notice the 'd' drops out). So, the stem of hidas is hitaa-, and adding -sti gives us hitaasti (slowly).

I thought hyvin meant "well". Why is it translated as "very" here?

You are right that hyvin is the adverb form of hyvä (good) and normally means "well". However, when you place hyvin directly in front of an adjective or another adverb (like hitaasti), it acts as an intensifier meaning "very". Therefore, hyvin hitaasti means "very slowly".

Could I use the verb toimia to say that a person is working?

No, toimia specifically means "to work" in the sense of a machine, product, or plan functioning properly. If you want to talk about a person working at a job or doing labor, you need to use the verb työskennellä or the phrase olla töissä.

The adjective here is kallis. If I wanted to add an ending to it, how does its stem change?