PBKDF2 is deliberately slow (and gets slower and safer with more iterations, so you want to make it as slow as you practically can). Because of this, you should generally avoid running it more times than you absolutely need to.
Using the same key and a different IV for each encrypted document is indeed OK — that's what the IV exists for. Most encryption schemes do have limitations on how much data and/or how many distinct messages you can safely encrypt with a single key, and you should check the documentation for the scheme you're using to make sure you're not at risk of exceeding these limits. But usually they're fairly generous, and if you limit yourself to, say, $2^{32}$ distinct documents per key, of up to $2^{32}$ cipher blocks each, you're probably fine. (In fact, you might need to worry more about message length limits than about message count limits, depending on which encryption mode you're using, especially if some of the documents you're encrypting might be larger than 64 GB.)
Alternatively, if you'd prefer to use a distinct key for each document, you can achieve that efficiently in (at least) two ways. One is to first derive a single master key from the password using PBKDF2 (with a high iteration count), and then derive the document keys from the master key using a faster KDF such as HKDF-Expand, with a distinct salt/info value for each document. The other method is to generate a random key for each document, encrypt that key using the master key (derived from the password using PBKDF2, as in the first method), and store it alongside the encrypted document.
Ps. Your question seemed familiar to me, so I checked to see if I might have answered something similar here before. While I didn't find any exact duplicates of your question, I did locate a couple of related questions that you might find useful, such as these:
