The thread I linked shows everything published about fupdate.
If you type: /fupdate
it shows the current value. Fupdate defaluts to 0 each time you start mirc, so any desired value should be in ON START or perform-on-connect. 100 is the maximum acceleration it does updating screen. In some cases you'll see the text blink and it's done, instead of a slow scroll. 0-99 are percentages of whatever 100 does, with 0 percent being 'off', the prior behavior.