Demonstration of slowdown when using BlitRect

test.c

#include "SDL.h"
#include "SDL_gpu.h"
#include <stdio.h>

#define WIDTH 280
#define HEIGHT 160
#define SCALE 3

bool running = true;
bool doScale = true;

GPU_Image *buffer;
GPU_Target *bufferTarget;
GPU_Target *renderer;

void loop() {
    SDL_Event event;
    while (SDL_PollEvent(&event)) {
        switch (event.type) {
        case SDL_QUIT:
            running = false;
            break;
        case SDL_KEYDOWN:
            if (event.key.keysym.sym == SDLK_ESCAPE)
                running = false;
            else
                doScale = !doScale;

            break;
        default:
            break;
        }
    }

    GPU_Clear(renderer);

    if (doScale) GPU_BlitRect(buffer, nullptr, renderer, nullptr);
    else GPU_Blit(buffer, nullptr, renderer, WIDTH / 2, HEIGHT / 2);

    GPU_Flip(renderer);
}

int main() {
    SDL_Init(SDL_INIT_VIDEO);

    SDL_Window *window = SDL_CreateWindow(
        "Test",
        SDL_WINDOWPOS_CENTERED,
        SDL_WINDOWPOS_CENTERED,
        WIDTH * SCALE, HEIGHT * SCALE,
        SDL_WINDOW_OPENGL
    );

    GPU_SetInitWindow(SDL_GetWindowID(window));
    renderer = GPU_Init(
        WIDTH * SCALE, HEIGHT * SCALE,
        GPU_DEFAULT_INIT_FLAGS
    );

    if (renderer == nullptr) {
        printf("Could not create window: %s\n", SDL_GetError());
        return 1;
    }

    buffer = GPU_CreateImage(WIDTH, HEIGHT, GPU_FORMAT_RGBA);
    GPU_SetBlending(buffer, GPU_FALSE);
    GPU_SetImageFilter(buffer, GPU_FILTER_NEAREST);

    bufferTarget = GPU_LoadTarget(buffer);
    SDL_Color color = {255, 0, 255, 255};
    GPU_ClearColor(bufferTarget, color);

    while (running) {
        int start = SDL_GetTicks();

        loop();

        int time = SDL_GetTicks() - start;
        if (time < 0) continue;

        int sleepTime = 100/6 - time;
        if (sleepTime > 0) {
            SDL_Delay(sleepTime);
        } else {
            SDL_Delay(1);
        }
    }

    GPU_FreeImage(buffer);
    GPU_FreeTarget(renderer);

    GPU_Quit();
    SDL_Quit();

    return 0;
}