There is no problem. They are both valid ways to apply for Canadian visas.

However, you may only use IRCC portal for visitor visas under certain circumstances (e.g. family members of a Canadian citizen or permanent resident, business visitors, compassionate reasons etc.). If you were directed by the website to use the old submission system, there is no problem at all.

> My understanding is the new website seems to offer faster processing times

Yes, but that is partly because the government wishes to prioritize people who are eligible to use the new portal (family members, business visitors, people who will attend funerals, certain students and workers) and partly due to updated technology (including the use of automated decision assistance services).

If you submitted your visa application very recently, there is no point to submit another application that may complicated the processing of your file.

If you submitted an eligible application a long time ago which has not been processed due to the pandemic backlog, the use of the new Portal may be considered.