Yes, partitioning will decrease a bit the read performance of queries not correlated with the partition key. That's why you need to periodically merge smaller partitions, so that you can keep the overall partition count bounded.

It is a lot of admin work, but if you really need to scale up Postgres write throughput, I don't see many other options without increasing hardware costs.

I assume you have already picked the low-hanging fruit discussed in the neighboring comments - batch writes, make sure you are using COPY instead of INSERT, tune Postgres parameters adequately and use the fastest disk you can grab for the WAL.