It will certainly affect your speed going up hill - but effectively not on the flat. The steeper the hill, the more difference it will make…

Zwift seems to be pretty realistic in the way it models climbs - in other words, if it takes you a certain amount of time to climb a hill with an average gradient of X in Zwift, it should take you about the same amount of time to climb a hill of the same gradient in “real life”, providing your power output is the same and you have entered your weight correctly.

The place to go to do cycling maths of this sort is the Analytic Cycling website (google it). If you select static forces / speed given power you can work out your speed up a hill of a given gradient. There are a whole bunch of assumptions and if you are interested I can give you a good set of defaults to enter, but the critical ones are slope (a 4% gradient (as in the Zwift Watopia epic KOM) = a slope of 0.04), weight (including bike, rider, all gear etc) and power. Oh, and the answer is in meters per second, so there is a bit of maths involved…

To cut a long story short, when I use this to calculate what my time should be for a 5.8 mile hill of average 4% gradient using my average power (260 W) from my best attempt on the Zwift epic KOM it gives me 23 minutes 54s, which is pretty close to my actual time on Zwift of 23m 16s. Doubtless I could realistically tweak the parameters to get it spot on. As you may have guessed I’m pretty light at 63kg, and when I do the sums on Analytic Cycling I enter 72kg, which is 63kg plus the weight of my best bike, clothing and equipment in real life. Zwift must do something similar and calculate a total weight for bike + rider etc by adding a few kilos to the rider weight entered.

Anyway - to answer your question - if I do the sums for myself and add an extra 2 kilos, it adds about 20 seconds to my theoretical time. So your weight loss from 96kg to 93.8kg can probably be expected to save you about that amount of time on that climb. It might not sound like a lot, but it is really when you think about it. E.g. 3kg weight loss would gain you about half a minute, which is a big margin at the competitive end of things, even for amateurs/mortals…