Does blockConcurrencyWhile() block the entire Durable Object?

export class MyDO extends DurableObject {
  async slowWrite() {
    return this.ctx.blockConcurrencyWhile(async () => {
      await fetch('https://slow-api.com'); // 15 seconds
      await this.ctx.storage.put('key', 'value');
    });
  }
  
  async fastRead() {
    return await this.ctx.storage.get('key');
  }
}

async fetch(request: Request): Promise<Response> {
  const url = new URL(request.url);
  
  if (request.method === 'POST' && url.pathname === '/write') {
    // WITH blocking
    return this.ctx.blockConcurrencyWhile(async () => {
      return this.slowWrite();
    });
  }
  
  if (request.method === 'GET' && url.pathname === '/read') {
    // WITHOUT blocking
    return this.fastRead();
  }
}