@DPOIRE You can certainly calculate as many values in timechart as you like and round them as needed, although rounding after a timechart+split_by needs to use "foreach" because the field names are named by the split field (in your case 'host')
| timechart span=5m avg(Delay) max(Delay) by host | foreach * [ eval "<<FIELD>>"=round('<<FIELD>>', 3) ]
Note that the left hand side of eval uses double quotes and the right hand side, single, round the <<FIELD>>.
That is to take care of any potential odd characters in the host results.
for my knowledge, you cannot calculate two values in timechart, so you need to use stats,
please try this:
my search | span span=5m _time | stats avg(Delay) AS avgDelay max(Delay) AS maxDelay by _time host | eval avgDelay=round(avgDelay,3), maxDelay=round(maxDelay,3)