My heart goes out to you - so very much.
My father passed away in October from PCNS lymphoma. Like your mother, he'd exhibited few symptoms (had some eye floaters that his eye doctor chalked up to "getting older") until one weekend, he had severe speech issues.
My first reaction was to think it was a stroke. I called the paramedics and by the time the MRI was done, we were told he had a mass in his brain with multiple lesions.
He responded well to initial treatment but the tumor, being so advanced, eventually grew resistent to chemo. From diagnosis to death, it was a turbulent eight months.
My father had little pain as well. It was a mercy. He simply grew weaker and less responsive as the days drew closer to the end. He passed peacefully. A single exhale and he was gone.
He looked more peaceful than I'd seen him in months.
I think we suffer (as caregivers) more than anyone. We can only sit back and watch as they slip away. It's really tough.
Spend as much time with her as you can. Tell her you love her, every chance you can.
I'd crawl in bed after my father became fully unresponsive, and just run my fingers through his hair and tell him how much I loved him. I made sure, at that point, to tell him it was okay to go too. I didn't want him to keep fighting just for me (we were each other's best friend).
I grieve for losing him, greatly. As an agnostic, I don't have the full comfort of faith, sadly. Some days, I wish I did though.
Surround her with all things great about her life. Pictures. Music. Family. All of it. Let her final moments be surrounded by utter love.
I'm so, so sorry for you, for your mother, and for your family.