MTU can have an effect. However, it typically does not have a huge impact unless it is unnecessarily low. For example, if it were dropped down to 1492 for PPPoE, that isn't a huge deal. However, what is a big deal is if there is a Path MTU Discover issue alongside a nonstandard MTU (typical is 1500). In that case, you can play with TCP MSS to determine if that is in fact an issue. I always set the MSS to 40 less than the MTU when I have to force down an MTU. For example--ip tcp adjust-mss 1452.
We've been using the tcp adjust-mss cmd where there are applications related issues or web page not loading correctly or applications running terribly slow. However this customer is using the internet and he is getting maximum 498~500 KB/sec download rate which is 4 Mbps, also his speedtest results show only 4.2 Mbps download speed.
Does MTU can make a really big difference ? I shall try tweaking the MSS parameter