Are you using an Apple Power Adapter - or a third-party alternative? Not all third-party Power Adapters are suitable for use with Apple devices - as they may use proprietary charging standards.
All Apple USB Power Adapters are designed for use with AC-power sources rated to provide 100VAC to 240VAC at 50Hz to 60Hz. Apple iPad USB power adapters also comply with the IEC/UL 60950-1 and IEC/UL 62368-1 standards for use with power sources rated to provide 115VAC at 400Hz (typically found onboard commercial airlines). As such, an Apple Power Adapter will work just as well in the US (~110V @60Hz) as it will in Europe/UK (220-240V @50Hz).
While a US-sourced (?) Power Adapter will be fully usable when travelling internationally, you may require a simple pin-adapter to allow the Power Adapter to be used with AC power outlets in different countries. For example, a US flat 2-pin connector will require an adapter for the standard European round 2-pin socket or UK 3-pin socket. Travel adapters are readily available.
These support pages should help with troubleshooting general battery and charging issues:
Charge and monitor the iPad battery - Apple Support
If your iPad won't charge – Apple Support
Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item. Most models of iPad require a Power Adapter rated at 12W or greater to reliably charge. 5W Power Adapters intended for older models of iPhone are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow.
Be aware that many computer USB ports cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, computer USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).
If you continue to experience difficulties when using an appropriate Power Adapter of 12W or greater, you might be best advised to seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad, Power Adapter and cable(s):
Genius Bar Reservation and Apple Support Options - Apple
As for your issues of poor performance, more details will be need. However, you mention having purchased an iPad with only 64GB of internal storage - this, by modern standards, is really not very much and can be easily filled. The Operating System and cached data will typically consume 10-15GB - before considering any documents, data and photos that you will likely save on your device. If you are close to filling your available storage, your iPad will become quite slow. You can check your available storage from iPad settings:
Settings > General > iPad Storage