Your internet speed is reliant on network congestion, prioritisation and coverage of your local network carrier.
If your eSIM’s APN was set up manually and is not set up correctly, it may appear as though your internet is slow. The real problem, in this case, is the incorrect installation of your eSIM.
Try to turn on airplane mode and then turn it off. If your eSIM supports multiple networks, try switching to a different network carrier.
If required, try to set up your APN settings or try switching to 3G data temporarily under Cellular > Voice and Data.